The present invention relates to analog and digital control circuitry and more particularly to an automatic gain control (AGC) for use with analog and digitally implemented receiving systems.
There are known in the prior art a variety of automatic gain control systems which are useful in the construction and operation of gain control in analog receivers. While such AGC control circuits have been successful in improving the operation of such receivers, there are situations in which the specific constructions of the analog or digitally implemented receivers are limited in operation by the specific configuration of the automatic gain control circuits. In addition, while an AGC provides the same functions in both an analog and digitally implemented system, there are significant differences (because of the differences in hardware) in the approach that must be used in the construction of such AGC circuits.
By way of example, in conventional analog systems, the response time and other characteristics of an AGC circuit can be improved by use of a log envelope detector. Such circuits are well-known in the art as exemplified by U.S. Pat. No. 4,263,560, and have been used successfully in analog circuits to control the gain of a variable gain amplifier receiving an analog RF input. In such analog systems, however, the overall cost of the analog radio as a result of the use of such log detectors is increased and has thus only been used where cost is not of great concern. Nevertheless, the log envelope detection technique of the type referenced above has applicability to both analog and digitally implemented systems because of the corresponding functional similarities in both systems.
In the presently-evolving trend toward implementation of high frequency receivers with a mixture of analog and digital techniques to optimize signal processing, similar control circuits (including the AGC control) may be implemented in a digital format. In moving to such corresponding digital control, conventional digital techniques are utilized to produce the desired automatic gain control. One of the benefits in implementing the digital AGC is that it is more economical to convert the signal level representations needed for log envelope detection in a digital environment to enable the production of a linear AGC using the techniques of the type referenced in U.S. Pat. No. 4,263,560. Accordingly, HF receivers employing a combination of analog and digital techniques are evolving which allow improved signal processing and the implementation of conventional techniques which have heretofore been unacceptable because of cost considerations.
While such analog/digital hybrid receivers have improved performance, the necessity for an A/D converter in going from an analog receiver to digital processing results in a requirement of more gain before the digital IF filter in the receiver. This means that appreciable AGC must be applied before the narrowband digital IF filter so that the A/D converter can operate satisfactorily. Accordingly, because of the envelope delay associated with such a narrow band filter prior to the log envelope detection, special techniques must be employed to provide the AGC with adequate speed while still controlling overshoot for proper operation. The same envelope delay in the IF filter of a totally analog system can also be overcome with those special techniques in order to improve the operation of a purely analog system.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide an improved automatic gain control circuit for analog and digitally implemented circuits.